Various types of vessels are commonly used for drilling wells offshore, including barges, jackups, drill ships, and semisubmersibles.
Semisubmersible vessels typically have a superstructure deck or decks supported by columns which are attached to hulls or pontoons which have adjustable ballast capability. By adjusting the ballast carried by the pontoons, the pontoons may be positioned at or near the surface of the water or in a submerged location below the surface of the water, while the superstructure deck remains above the surface of the water. While being moved to a location where the vessel is to be used, the pontoons typically are ballasted to permit them to ride at or near the surface of the water, facilitating transport of the vessel. After reaching the desired location, the ballast of the pontoons may be adjusted to cause the pontoons to become submerged below the surface of the water, providing improved stability and reduced motion of the vessel in rough, deep seas. The vessel may or may not be self-propelled.
The pontoons in turn support the superstructure deck by columns which rise vertically or substantially vertically from the ring pontoon at various locations, as well as braces which may interconnect the pontoons, the pontoons and the columns, the columns and the superstructure, and/or two or more of the columns. The purpose is to provide a strong and substantially rigid base structure to support the deck(s) of the superstructure.
The interior of both the columns and the pontoons may be subdivided by bulkheads to strengthen the structure, to provide enclosed spaces for locating and storing various equipment (e.g., anchors, chains, propulsion mechanisms, etc.), and to provide a plurality of separate tanks for purposes of ballasting the vessel and storing various fluids and other materials which may be required or desired during drilling or produced by the well.
The columns typically have been placed such that the vertical centerline of the columns intersects the axial centerline of the starboard or port pontoon on which the column is located. In a ring pontoon semisubmersible vessel previously proposed, the four corner columns have been placed such that the vertical centerline of the columns intersects the axial centerline of the forward or aft pontoon as well.